1. Field of Art
This invention relates to helicopter control systems, and more particularly to reduction of helicopter oscillations sustained by the helicopter automatic control system.
2. Description of the Prior Art
It is known in the art to provide helicopters with automatic flight control systems (AFCS) which sense various parameters, such as attitude, velocity and acceleration in various axes, to stablize the aircraft both in terms of long-term flight path (autopilot) and in terms of short-term undesirable perturbations (such as may be induced by wind gusts and the like). The utilization of automatic flight control necessarily provides a closed loop system which includes the aircraft response and the commands generated in response to sensors which indicate the aircraft response, thereby inducing changes in the aircraft response. Necessarily, being a closed loop system, the opportunity for creation of oscillatory interaction always exists. In the past, various mechanical characteristics of helicopters and their control systems have tended to alleviate any tendency for oscillatory coupling between the helicopter and its automatic flight control systems. For instance, the main rotor blade system of the typical helicopter has very large inertia and includes a fair amount of damping. Mechanical play in linkage systems masks many small responses. The general transfer characteristic of the automatic flight control command in response to changes in aircraft attitude and position (including gain and frequency response) also may be limited so as to avoid oscillation. In some automated control systems, trim valves may operate as hydraulic dampers when not providing trim correction input commands to the helicopter. And, mass-balancing of mechanical controls may provide specific oscillation compensation.
In helicopters capitalizing on new technology, the rotor blades may be lower in size and weight and therefore provide a much lower inertia, thereby requiring less mechanical damping to be associated therewith. Similarly, the rotor blades themselves are capable in such a case of responding more rapidly to commands applied thereto. Mass-balancing adds weight and slows response, and may therefore be undesirable. Demands for helicopters having improved performance response characteristics, with tighter linkages and higher gains and frequency response in the automated control systems, are therefore much more prone toward oscillation induced by coupling between the helicopter response and the automatic flight control system commands provided thereto.